Instead of anchoring both scrollview and weapon Selection to L anchor and R anchor, have Weapon Selection anchor to L/R anchor and have scrollview anchor to weapon selection instead.
That ought to fix your problem.
I did a similar thing in the example 14 Endless scrollview that comes with NGUI, to make the third scroll fill the window, while the window was anchored to UIRoot to make it full screen, and that works just fine, even when I resize the game view.